1. Field of the Invention
The present invention relates to a method of packaging various types of electronic components on a printed circuit board (PCB) by means of soldering, and in particular, to a method of packaging electronic components on a PCB using a reflow soldering method.
2. Description of the Related Art
Soldering is conventionally employed as the method of packaging electronic components on a printed circuit board (PCB). Referring now to FIG. 1, one example of a method of packaging an electronic component using soldering is explained. Explanation is here presented taking as an example double-sided reflow in which electronic components are soldered onto both sides of a PCB by reflow.
Solder paste is first printed on lands that are provided on the PCB (Step 101). A summary of this printing process is shown in FIGS. 2A to 2C.
In this process, metal mask 13 is first arranged on PCB 15 as shown in FIG. 2A. Openings 12 of the same dimensions as lands 14 are formed in metal mask 13 in a pattern of arrangement that corresponds to the pattern of arrangement of lands 14 that are provided on the PCB. Metal mask 13 is then arranged such that openings 12 of metal mask 13 are each positioned on respective lands 14 of PCB 15.
Solder 11 is then rolled over metal mask 13 by means of printing squeegee 10, whereby solder 11 fills openings 12 of metal mask 13 as shown in FIG. 2B. After openings 12 are filled with solder 11, metal mask 13 is removed from PCB 15 as shown in FIG. 2C. Solder paste is thus printed on lands 14.
Chip components, for example, a surface-mount component such as a QFP (Quad Flat Package) or an SOP (Small Outline Package), are next placed on the printed solder paste (Step 102). The PCB with the mounted surface-mount component is then passed through a high-temperature reflow furnace, whereby the solder paste is melted and the leads of the surface-mount component and the lands of the PCB are soldered together (Step 103). The processes to this point complete the packaging on one surface of the PCB. The PCB is then turned over such that the surface on which a component has not yet been packaged is directed upward (Step 104).
Solder paste is then printed (Step 105) and a component is mounted (Step 106) as in Steps 101 and 102. Through-hole (T/H) components, which is a component constructed for mounting by passing leads into T/H that are formed in the PCB, is then mounted (Step 107). The PCB is next passed through a reflow furnace as in Step 103 to solder the components (Step 108). Finally, components that are unable to withstand the high temperature of the reflow furnace are soldered by manual operation (Step 109). The packaging of electronic components onto a PCB is thus completed.
In the above-described conventional method of packaging electronic components, a tin-lead (Sn—Pb) solder is typically used as solder 11. This tin-lead solder, however, contains the toxic heavy metal lead (Pb). The use of lead raises the problem of damage to the global environment that results from inappropriate disposal of used electronic equipment. As a solution to this problem, the use of lead-free solder that does not contain lead (Pb) has been preferred in recent years to prevent pollution of the environment.
Tin-silver (Ag) solder is well known as such a lead-free solder. Due to the stable characteristics of silver, the use of tin-silver solder in place of tin-lead solder for packaging electronic components enables packaging of electronic components that maintains the same level of reliability as in the prior art. However, the melting point of tin-silver solder is approximately 220° C., which is far higher than the approximately 183° C. melting point of tin-lead solder. For this reason, problems are encountered when tin-silver solder is simply substituted for tin-lead solder in packaging that was performed using tin-lead solder. Problems are also encountered when equipment that was used to carry out packaging with tin-lead solder is used without modification to carry out packaging while using tin-silver solder. In particular, when soldering by melting tin-silver solder having a melting point of 220° C. in a reflow furnace, electronic components may in some cases reach temperatures in excess of 240° C. The temperature endurance of a typical electronic component is approximately 230° C. Thus, when tin-silver solder is used to package electronic components, there arises the problem that the temperature endurance of the various electronic components that are used must be raised.
Besides tin-silver solder, which has a high melting point, tin-zinc (Sn—Zn) solder is also known as a lead-free solder. The melting point of this tin-zinc solder is approximately 197° C., and this tin-zinc solder can therefore be used to package electronic components of the prior art using the equipment of the prior art without modification.
A case is here considered in which, as the electronic component, a device having L-type leads such as a tantalum capacitor or a leadless chip component ranging in size from the 1608 size (0.8 mm×1.6 mm) to the 5750 size (5.7 mm×5 mm) is mounted on a PCB.
As described hereinabove, openings 12 of metal mask 13 usually have the same dimensions as lands 14. If, after using such a metal mask 13 to print solder paste on PCB 15, a device having L-type leads or a leadless chip having electrodes that occupy a relatively large region of lands 14 is mounted on PCB 15, a portion of solder 11 that has been printed on lands 14 is in some cases pressed out into an overflow region. A portion of solder 11 that has been pressed out from lands 14 in this way is absorbed by solder 11 on lands 14 in the reflow process, but a portion of this solder 11 may form solder balls in areas outside lands 14. If such solder balls are formed, they may cause breakdowns of PCB 15.
In particular, since the wettability of the above-described lead-free tin-zinc (Sn—Zn) solder is poorer than solder paste that contains lead (Pb), the use of this lead-free solder tends to result in the formation of the above-described solder balls. As a result, a countermeasure for preventing the occurrence of solder balls is all the more desirable when lead-free tin-zinc (Sn—Zn) solder is used.